Base-Promoted Silicon−Hydrogen Bond Activation of Silanes by

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Organometallics 2008, 27, 4034–4042

Articles Base-Promoted Silicon-Hydrogen Bond Activation of Silanes by Iridium(III) Porphyrin Complexes Baozhu Li and Kin Shing Chan* Department of Chemistry, The Chinese UniVersity of Hong Kong, Shatin, New Territories, Hong Kong, People’s Republic of China ReceiVed NoVember 14, 2007

Iridium(III) porphyrin silyls were synthesized in moderate to high yields conveniently from the reactions of iridium(III) porphyrin carbonyl chloride and methyl with silanes, via silicon-hydrogen bond activation (SiHA) in solvent-free conditions and nonpolar solvents. Base was found to promote the SiHA reactions. Specifically, K3PO4 accelerated the SiHA with iridium porphyrin carbonyl chloride, while KOAc promoted the SiHA by iridium porphyrin methyl. Mechanistic experiments suggested that iridium(III) porphyrin carbonyl chloride initially formed iridium porphyin cation, which then reacted with silanes likely via heterolysis to give iridium porphyrin hydride. Iridium porphyrin hydride further reacted with silanes to yield iridium porphyrin silyls. On the other hand, iridium(III) porphyrin methyl and silyls underwent either oxidative addition or σ-bond metathesis to form the products. In the presence of base, a pentacoordinated silicon hydride species likely formed and reacted with iridium porphyrin methyl to form iridium porphyrin anion, which could further react with silanes to yield iridium porphyrin hydride after protonation. Ir(ttp)H finally reacted with excess silanes to give iridium porphyrin silyl complexes. Introduction Silicon-hydrogen bond activation (SiHA) is an important process for both catalytic applications in organic synthesis1–3 andthesynthesisoftransitionmetalsilyls.4 Recently,silicon-hydrogen bond activation has been utilized for the catalytic functionalization of hydrocarbons by transition metal complexes, and transition metal silyls are proposed as the intermediates.2,3 The preparation of transition metal silyls from the reactions of metal halides with silanes has been extensively utilized.5 The silicon-hydrogen bond activation with the late highvalent transition metal such as rhodium(III) or iridium(III) complexes is interesting.5–7 So far, the mechanistic possibilities of silicon-hydrogen bond activation by these late transition metal complexes can be heterolysis,8,9 oxidative addition,7,10,11 * Corresponding author. E-mail: [email protected]. (1) (a) Yang, J.; Brookhart, M. J. Am. Chem. Soc. 2007, 129, 12656– 12657. (b) Yamanoi, Y.; Taira, T.; Sato, J.-I.; Nakamula, I.; Nishihara, H. Org. Lett. 2007, 22, 4543–4546. (2) Tsukada, N.; Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 5022– 5023. (3) Yamanoi, Y.; Nishihara, H. Tetrahedron Lett. 2006, 47, 7157–7161. (4) (a) Casty, G. L.; Tilley, T. D.; Yap, G. P. A.; Rheingold, A. L. Organometallics 1997, 16, 4746–4754. (b) Turculet, L.; Feldman, J. D.; Tilley, T. D. Organometallics 2004, 23, 2488–2502. (c) Mork, B. V.; Tilley, T. D.; Schultz, A. J.; Cowan, J. A. J. Am. Chem. Soc. 2004, 126, 10428– 10440. (5) Corey, J. Y.; Braddock-Wilking, J. Chem. ReV. 1999, 99, 175–292. (6) Taw, F. L.; Bergman, R. G.; Brookhart, M. Organometallics 2004, 4, 886–890. (7) Karshtedt, D.; Bell, A. T.; Tilley, T. D. Organometallics 2006, 25, 4471–4482. (8) Zhang, L. R.; Chan, K. S. Organometallics 2006, 25, 4822–4829. (9) Klei, S. R.; Tilley, T. D.; Bergman, R. G. Organometallics 2002, 21, 3376–3387.

or σ-bond metathesis.12,13 One key supporting evidence for the oxidative addition is the isolation of an iridium(V) silyl intermediate, which has been characterized by X-ray diffraction.7,11 Our group has reported that silicon-hydrogen bonds activation by rhodium(III) porphyrins (por) can provide a facile synthetic route to rhodium silyl complexes.8 We have reported that rhodium(III) porphyrin halides and methyls reacted with silanes to give high yields of rhodium porphyrin silyls via silicon-hydrogen bond activation in solvent-free conditions. Rh(por)Cl was proposed to undergo ionization into Rh(por)+Cl-, which then underwent silylation to yield rhodium porphyrin silyls. On the other hand, Rh(por)Me reacted with silanes via either oxidative addition or σ-bond metathesis (Scheme 1). These reactions are mechanistically puzzling since cis-interaction on the same face of a porphyrin by the methyl group and silane could be sterically very demanding. To broaden the synthetic scope and to gain further mechanistic understanding of these metalloporphyrin-based SiHA, we have explored the SiHA chemistry with high-valent iridium(III) porphyrins. In addition, the base-promoted SiHA was discovered. Herein, we now report the successful results of these studies. (10) Fernandez, M. J.; Maitlis, P. M. J. Chem. Soc., Dalton Trans. 1984, 3, 2063–2066. (11) Klei, S. R.; Tilley, T. D.; Bergman, R. G. J. Am. Chem. Soc. 2000, 122, 1816–1817. (12) (a) Radu, N. S.; Tilley, T. D. J. Am. Chem. Soc. 1995, 117, 5863– 5864. (b) Ziegler, T.; Folga, E. J. Organomet. Chem. 1994, 478, 57–65. (c) Woo, H.-G.; Walzer, J. F.; Tilley, T. D. J. Am. Chem. Soc. 1992, 114, 7047–7055. (13) Perutz, R. N.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2007, 46, 2578–2592.

10.1021/om701144a CCC: $40.75  2008 American Chemical Society Publication on Web 07/22/2008

Silicon-Hydrogen Bond ActiVation of Silanes

Organometallics, Vol. 27, No. 16, 2008 4035

Scheme 1. Mechanism of SiHA of Silanes with Rh(ttp)Cl or Rh(ttp)Me

Table 2. Solvent-Free SiHA of Silanes by Ir(ttp)Cl(CO)

entry 1 2 3 4 5 6 7a

HSiR3

temp (°C)

time

product (% yield)b

HSiEt3, 2a HSi(OEt)3 3a HSiBnMe2 4a H3SiPh 5a H2SiPhMe 6a H2SiPh2 7a HSiPh2Me 8a

200 200 200 140 200 140 200

4 days 6h 1 day 5h 0.5 h 2 days 6h

Ir(ttp)SiEt3 2b (84) Ir(ttp)Si(OEt)3 3b (54) Ir(ttp)SiBnMe2 4b (78) Ir(ttp)SiPhH2 5b (90) Ir(ttp)SiPhMeH 6b (34) Ir(ttp)SiPh2H 7b (68) Ir(ttp)SiPh2Me 8b (42)

a 30% yield of Ir(ttp)SiPhMeH 6b was also generated according to H NMR analysis of the crude reaction mixture. b Average yield of duplicate experiments. 1

Optimization in Solvent-Free Conditions. Initially, Ir(ttp)Cl(CO) 1a (Figure 1) reacted with HSiEt3 2a in solvent-free conditions at 120 °C (Table 1, entry 1). However, only a trace amount of Ir(ttp)SiEt3 2b was obtained even after 9 days, while Ir(ttp)H 1d was observed as the major product according to the 1H NMR analysis of the crude reaction mixture. When the reaction temperature was increased to 200 °C, after 4.5 h, Ir(ttp)H was generated as the major product together with Ir(ttp)SiEt3 2b isolated in a low yield of 6% (Table 1, entry 3). After 4 days, Ir(ttp)SiEt3 2b was isolated in 84% yield (Table 1, entry 4). Therefore, the temperature of 200 °C was employed for subsequent studies. When the optimized solvent-free conditions at 200 °C were applied to the reactions of Ir(ttp)Cl(CO) 1a with various silanes, moderate to good yields of iridium porphyrin silyl complexes were obtained (eq 1, Table 2). Ir(ttp)Cl(CO) 1a dissolved in alkyl-substituted silanes only upon heating, but dissolved in aromatic silanes even at room temperature. The yields of the products depended on the steric hindrance of silanes.8 Cone angles obtained from phosphines have been used as a measure of steric hindrance, and cone angles of silanes are identical to the corresponding phosphines14 [H3SiPh (101°), HSi(OEt)3 (109°), H2SiPh2 (128°), HSiEt3 (132°), HSiPr3 (132°), HSiPh2Me (136°)].15 In general, more bulky silanes with larger cone angles reacted with slower rates and gave lower yields. No SiHA product was found when Ir(ttp)Cl(CO) 1a reacted with

the more hindered HSiPr3 or HSitBuMe2. However, the rate of SiHA was also dependent on the solubility of Ir(ttp)Cl(CO) 1a in silane. A better solvent, such as H2SiPhMe or HSiPhMe2, gave a faster rate (Table 2, entries 5 and 7), while a poor solvent such as HSiEt3 gave a slower rate (Table 2, entry 1). Iridium porphyrin silyls are thermally stable when the silyl group is trisubstituted. However, the monosubstituted Ir(ttp)SiPhH2 5b decomposed at 200 °C even in a nitrogen atmosphere. Therefore, the reaction of Ir(ttp)Cl(CO) 1a and H3SiPh 5a was carried out at a lower temperature of 140 °C (Table 2, entry 4). Ir(ttp)SiPhH2 5b and Ir(ttp)SiPhMeH 6b could not be purified by column chromatography on alumina or recrystallization from CH2Cl2/MeOH. However, these two complexes could be purified simply by washing off the impurities with methanol under N2. ForHSiPh2Me8aonly,besidestheSiHAreaction,silicon-carbon bond activation (SiCA) was also observed. When 8a reacted with Ir(ttp)Cl(CO) 1a at 200 °C, according to 1H NMR analysis, Ir(ttp)H16,17 initially formed. Then both the SiHA product 8b and SiCA product 6b were generated slowly and simultaneously in the ratio of 1.00 to 1.16 with the slow disappearance of Ir(ttp)H.18 In addition, Ir(ttp)H also reacted with HSiPh2Me to generate both 8b and 6b at 200 °C in about 1.00 to 0.71 ratio,18 which suggested that Ir(ttp)H was the intermediate for the parallel formation of both 8b and 6b in the reaction of 1a and 8a at 200 °C. Base-Promoted SiHA with Ir(ttp)Cl(CO). Yamanoi and his co-worker reported that a base, especially K3PO4, can promote catalysis of a SiHA reaction.3 Our group also found that 2,6dimethylpyridine can increase the rate of SiHA of Rh(ttp)X (X ) Cl and OTf) and HSiEt3.8 We therefore examined the possibility of base-promoted SiHA. Table 3 and eq 2 show the successful results of base-promoted SiHA reaction of Ir(ttp)Cl(CO) 1a and HSiEt3 2a in solventfree conditions. Addition of 5 equiv of K3PO4 enhanced the reaction rate and yield slightly (Table 3, entries 3 and 2), while addition of 10 equiv of K3PO4 enhanced the reaction rate and yield remarkably even at a lower temperature of 140 °C (Table 3, entry 4). However, the addition of 20 equiv of K3PO4 decreased the reaction yield slightly (Table 3, entry 5), and this may be due to the decomposition of intermediates or the occurrence of a side reaction caused by excess base. The optimal amount of K3PO4 was therefore found to be 10 equiv. We further screened other salts with anions of different basicity and nucleophilicity in the reaction of Ir(ttp)Cl(CO) and HSiEt3. The pKa values for conjugated acids have been used to

(14) Tolman, C. A. Chem. ReV. 1977, 77, 313–348. (15) (a) Hester, D. M.; Sun, S.; Harper, A. W.; Yang, G. K. J. Am. Chem. Soc. 1992, 114, 5234–5240. (b) Goikhman, R.; Milstein, D. Chem.Eur. J. 2005, 11, 2983–2988.

(16) Yeung, S. K.; Chan, K. S. Organometallics 2005, 24, 6426–6430. (17) Wayland, B. B.; Van Voorhees, S. L.; Wilker, C. Inorg. Chem. 1986, 25, 4039–4042. (18) See the Supporting Information for details.

Figure 1. Structures of iridium porphyrin complexes. Table 1. Temperature Optimization of SiHA entry 1 2 3 4 a

temp (°C)

time

120 140 200 200

9 days 6 days 4.5 h 4 days

% 2ba trace 10 6 84

Average yield of duplicate experiments.

Results and Discussion

4036 Organometallics, Vol. 27, No. 16, 2008

Li and Chan

Table 3. Base-Promoted SiHA of HSiEt3 in Solvent-Free Conditions

entry

base

equiv

temp (°C)

time (days)

% 2ba

1 2 3 4 5 6 7 8 9 10 11

none none K3PO4 K3PO4 K3PO4 K2CO3 KOH KF CsCl KI KOAc

0 0 5 10 20 10 10 10 10 10 10

200 140 140 140 140 140 140 140 140 140 140

4 6 1 1 1 1 1 5 6 5 5

84 10 46 88 80 16 37 50 4 14 12

a

Scheme 3. Mechanism of SiHA and SiCA of Ir(ttp)Cl(CO) and HSiPh2Me

Table 4. SiHA of Silanes by Ir(ttp)Cl(CO)

Average yield of duplicate experiments.

Scheme

2.

Mechanism of Base-Promoted Ir(ttp)Cl(CO) with HSiEt3

SiHA

of

entry 1 2 3 4 a

compare the basicities of anions [H2O (15.7) > H3PO4 (12.32) > H2CO3 (10.3) > HOAc (4.76) > HF (3.17) > HCl (-8.0) > HI ( 2σ(I)] final R1a/wR2b (all data) w1/w2c a R1 ) ∑(||Fo| - |Fc||)/∑|Fo|. 2Fc2)/3.

b

3b

4b

C54 H51Ir N4O3Si triclinic P1j 1024.28 11.743 (6) 12.886 (6) 16.755 (8) 83.320 (10) 84.748 (9) 85.872 (10) 2 1.359 2.736 2503 (2) 1036 0.50 × 0.40 × 0.30 13 356 SADABS 1.0000 and 0.480014 full-matrix least-squares on F2 8740/16/577 1.072 0.0545/0.1436 0.0677/0.1555 0.1018/0.8042

C57H49IrN4Si · 1/4CH2Cl2 monoclinic P21/c 1031.52 13.780 (2) 51.702 (9) 15.311 (3) 90 97.874 (4) 90 8 1.268 2.555 10806 (3) 4164 0.50 × 0.40 × 0.30 57 969 SADABS 1.0000 and 0.500568 full-matrix least-squares on F2 19 016/156/1216 1.138 0.0986/0.2632 0.1371/0.2851 0.1040/232.8366

wR2 ) {∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2.

Table 8. Selected Bond Lengths and Angles of Ir(ttp)Si(OEt)3 Ir(1)-N(1) Ir(1)-N(3) Ir(1)-Si(1) Si(1)-O(2) N(1)-Ir(1)-Si(1) N(3)-Ir(1)-Si(1) O(1)-Si(1)-Ir(1) O(3)-Si(1)-Ir(1) O(1)-Si(1)-O(3)

Bond Lengths (Å) 2.040 (6) Ir(1)-N(2) 2.029 (6) Ir(1)-N(4) 2.277 (3) Si(1)-O(1) 1.616 (9) Si(1)-O(3) Bond Angles (deg) 90.88 (19) N(2)-Ir(1)-Si(1) 94.3 (2) N(4)-Ir(1)-Si(1) 112.7 (5) O(2)-Si(1)-Ir(1) 113.3 (4) O(1)-Si(1)-O(2) 113.6 (8) O(3)-Si(1)-O(2)

2.022 (6) 2.029 (6) 1.552 (11) 1.604 (10) 95.1 (2) 92.9 (2) 109.6 (3) 103.9 (8) 102.7 (6)

Table 9. Selected Bond Lengths and Angles of Ir(ttp)SiBnMe2 Ir(1)-N(1) Ir(1)-N(3) Ir(1)-Si(1) Si(1)-C(62)

Bond Lengths (Å) 2.036 (12) Ir(1)-N(2) 2.025 (13) Ir(1)-N(4) 2.328 (5) Si(1)-C(61) 1.852 (6) Si(1)-C(63)

2.026 (14) 2.030 (13) 1.865 (2) 1.85 (3)

Bond Angles (deg) N(1)-Ir(1)-Si(1) 93.5 (4) N(2)-Ir(1)-Si(1) 92.3 (5) N(3)-Ir(1)-Si(1) 92.7 (4) N(4)-Ir(1)-Si(1) 91.6 (4) C(61)-Si(1)-Ir(1) 110.4 (8) C(62)-Si(1)-Ir(1) 110.8 (9) C(63)-Si(1)-Ir(1) 110.2 (8) C(61)-Si(1)-C(62) 106.6 (14) C(61)-Si(1)-C(63) 109.4 (15) C(63)-Si(1)-C(62) 109.8 (14)

anion. Ir(ttp)H was produced after protonation and finally reacted with excess silane to give iridium porphyrin silyl complexes.

Experimental Section Unless otherwise noted, all chemicals were obtained from commercial suppliers and used before purification. Hexane for chromatography was distilled from anhydrous calcium chloride. Thin-layer chromatography was performed on precoated alumina plates for analysis of reaction mixture. All reactions were carried out in a Teflon screw-head stoppered tube in N2 in the absence of light by aluminum foil wrapping. The purification of iridium silyl complexes was carried out by flash column chromatography in air using aluminum (90 active neutral, 70-230 mesh). All reactions

c

Weighting scheme w-1 ) σ2(Fo2) + (w1P)2 + w2P, where P ) (Fo2 +

were run in duplicate, and the yields obtained were average yields. Samples for microanalysis were recrystallized from CH2Cl2/MeOH and were then dried at 40-60 °C under vacuum (0.005 mmHg) for 3 days. Ir(ttp)Cl(CO)16 1a, Ir(ttp)Me16,17 1b, Ir(ttp)(BF4)(CO)21 1c, and Ir(ttp)H16,17 1d were prepared according to the literature. 1

H NMR spectra were recorded on a Bruker DPX-300 (300 MHz). Chemical shifts were reported with reference to the residual solvent protons in CDCl3 (δ 7.26 ppm) as the internal standard. Chemical shifts (δ) were reported in parts per million (ppm) downfield from TMS. Coupling constants (J) are reported in hertz (Hz). 13C NMR spectra were recorded on a Bruker DPX 300 (75 MHz) spectrometer and referenced to CDCl3 (δ 77.1 ppm) spectra. High-resolution mass spectra (HRMS) were performed on a Thermofinnign MAT 95 XL in FAB (using 3-nitrobenzyl alcohol (NBA) matrix and CH2Cl2 as the solvent) and ESI model (MeOH/ CH2Cl2, 1:1, as the solvent). Reactions of Ir(ttp)Cl(CO) 1a with Silanes. General Procedure. The reaction of Ir(ttp)Cl(CO) 1a with triethylsilane (2a) is described as a typical example. Triethylsilane (1.1 mL, 500 equiv) was added to Ir(ttp)Cl(CO) (12.5 mg, 0.014 mmol), and the mixture was degassed by the freeze-pump-thaw method (3 cycles). Then the mixture was heated at 200 °C for 4 days in N2. The crude product was purified by column chromatography on alumina eluting with hexane. A purple solid of Ir(ttp)SiEt3 2b (11.5 mg, 0.0118 mmol, 84%) was produced. Rf ) 0.52 (hexane/CH2Cl2 ) 5:1). 1H NMR (CDCl3, 300 MHz): δ -3.22 (q, 6 H, J ) 7.8 Hz), -1.25 (t, 9 H, J ) 7.8 Hz), 2.68 (s, 12 H), 7.50 (d, 8 H, J ) 8.4 Hz), 7.96 (t, 8 H, J ) 6.0 Hz), 8.51 (s, 8 H). 13C NMR (CDCl3, 75 MHz): δ 0.1, 5.3, 21.7, 29.9, 124.8, 127.6, 127.7, 131.3, 133.4, 133.8, 137.3, 138.8, 143.8. HRMS (FAB): calcd for (C54H51N4SiIr)+ m/z 976.3507; found m/z 976.3526. Anal. Calcd for C54H51N4SiIr: C, 66.43; H, 5.26; N, 5.74. Found: C, 66.43; H, 5.24; N, 5.64. Reaction between Ir(ttp)Cl(CO) 1a and Triethylsilane (2a). Triethylsilane (1.0 mL, 500 equiv) and Ir(ttp)Cl(CO) (11.2 mg, 0.012 mmol) were heated at 120 °C for 9 days. Only trace amount of Ir(ttp)SiEt3 2b was observed according to 1H NMR. Reaction between Ir(ttp)Cl(CO) 1a and Triethylsilane (2a). Triethylsilane (1.0 mL, 500 equiv) and Ir(ttp)Cl(CO) (11.2 mg, 0.012 mmol) were heated at 200 °C for 4.5 h. A purple solid of

4040 Organometallics, Vol. 27, No. 16, 2008 Ir(ttp)SiEt3 2b (0.7 mg, 0.0007 mmol, 6%) was obtained after column chromatography. Reaction between Ir(ttp)Cl(CO) 1a and Triethylsilane (2a). Triethylsilane (1.2 mL, 500 equiv) and Ir(ttp)Cl(CO) (14.1 mg, 0.015 mmol) were heated at 140 °C for 6 days under N2. A purple solid of Ir(ttp)SiEt3 2b (1.5 mg, 0.0015 mmol, 10%) was obtained after column chromatography. Reaction between Ir(ttp)Cl(CO) 1a and Triethoxysilane (3a). Triethoxyslane (1.3 mL, 500 equiv) and Ir(ttp)Cl(CO) (13.3 mg, 0.014 mmol) were heated at 200 °C for 6 h under N2. A purple solid of Ir(ttp)Si(OEt)3 3b (7.7 mg, 0.0076 mmol, 54%) was obtained after column chromatography. Rf ) 0.28 (hexane/CH2Cl2 ) 5:1). 1H NMR (CDCl3, 300 MHz): δ -0.22 (t, 9 H, J ) 6.9 Hz), 0.91 (q, 6 H, J ) 6.9 Hz), 2.68 (s, 12 H), 7.50 (d, 8 H, J ) 8.1 Hz), 7.99 (d, 8 H, J ) 8.1 Hz), 8.58 (s, 8 H). 13C NMR (CDCl3, 75 MHz): δ 17.3, 21.8, 55.1, 124.3, 127.7, 127.8, 131.2, 133.9, 137.4, 139.2, 143.6. HRMS (FAB): calcd for (C54H51N4SiIr)+ m/z 1024.3354; found m/z 1024.2260. Anal. Calcd for C54H51N4O3SiIr: C, 63.32; H, 5.02; N, 5.47. Found: C, 63.15; H, 4.98; N, 5.38. Reaction between Ir(ttp)Cl(CO) 1a and Benzyldimethylsilane (4a). Benzyldimethylsilane (1.2 mL, 500 equiv) and Ir(ttp)Cl(CO) (13.6 mg, 0.015 mmol) were heated at 200 °C for 1 day under N2. A purple solid of Ir(ttp)SiBnMe2 4b (11.8 mg, 0.0117 mmol, 78%) was obtained after column chromatography. Rf ) 0.55 (hexane/CH2Cl2 ) 5:1). 1H NMR (CDCl3, 300 MHz): δ -3.83 (s, 6 H), -2.21 (s, 2 H), 2.69 (s, 12 H), 5.32 (t, 2 H, J ) 3.6 Hz), 6.47 (t, 3 H, J ) 3.6 Hz), 7.50 (t, 8 H, J ) 7.2 Hz), 7.96 (d, 4 H, J ) 7.5 Hz), 8.04 (d, 4 H, J ) 7.5 Hz), 8.58 (s, 8 H). 13C NMR (CDCl3, 75 MHz): δ -7.3, 21.7, 21.1, 123.2, 125.2, 127.5, 128.1, 128.2, 131.8, 133.9, 134.3, 137.9, 139.2, 144.0. HRMS (FAB): calcd for (C57H49N4SiIr)+ m/z 1010.3350; found m/z 1010.3340. Anal. Calcd for C54H51N4O3SiIr: C, 67.76; H, 4.89; N, 5.54. Found: C, 67.65; H, 4.948; N, 5.41. Reaction between Ir(ttp)Cl(CO) 1a and Phenylsilane (5a). Phenylsilane (1.0 mL, 500 equiv) and Ir(ttp)Cl(CO) (15.6 mg, 0.017 mmol) were heated at 140 °C for 5 h under N2. A dark brown solid of Ir(ttp)SiH2Ph 5b (14.8 mg, 0.0153 mmol, 90%) was obtained when the reaction mixture was washed by MeOH. 1H NMR (CDCl3, 300 MHz): δ -2.16 (s, 2 H, 1JSi-H ) 202.8 Hz), 2.69 (s, 12 H), 4.33 (d, 2 H, J ) 6.3 Hz), 6.37 (t, 2 H, J ) 6.3 Hz), 6.76 (t, 1 H, J ) 6.3 Hz), 7.51 (d, 8 H, J ) 7.5 Hz), 7.88 (d, 4 H, J ) 7.2 Hz), 8.00 (d, 4 H, J ) 7.2 Hz), 8.53 (s, 8 H). 13C NMR (CDCl3, 75 MHz): δ 21.7, 124.2, 126.1, 127.5, 127.6, 131.3, 131.4, 133.6, 133.9, 137.3, 138.7, 143.0. HRMS (FAB): calcd for (C54H43N4SiIr)+ m/z 968.2881; found m/z 968.2872. Reaction between Ir(ttp)Cl(CO) 1a and Phenylmethylsilane (6a). Phenylmethylsilane (0.8 mL, 500 equiv) and Ir(ttp)Cl(CO) (11.2 mg, 0.012 mmol) were heated at 200 °C for 0.5 h under N2. A purple solid of Ir(ttp)SiPhMeH 6b (4.0 mg, 0.0041 mmol, 34%) was obtained when the reaction mixture was washed by MeOH. Rf ) 0.46 (hexane/CH2Cl2 ) 5:1). 1H NMR (CDCl3, 300 MHz): δ -3.45 (d, 3 H, J ) 3.3 Hz), -2.24 (q, 1 H, J ) 3.3 Hz, 1JSi-H ) 200.1 Hz), 2.68 (s, 12 H), 4.34 (d, 2 H, J ) 6.9 Hz), 6.40 (t, 2 H, J ) 7.5 Hz), 6.78 (t, 1 H, J ) 7.4 Hz), 7.53 (t, 8 H, J ) 5.7 Hz), 7.86 (d, 4 H, J ) 8.4 Hz), 7.98 (d, 4 H, J ) 8.4 Hz), 8.49 (s, 8 H). 13 C NMR (CDCl3, 75 MHz): δ -9.8, 21.8, 124.5, 126.2, 127.5, 127.7, 127.8, 130.6, 131.4, 133.7, 134.0, 137.4, 138.9, 143.3. HRMS (FAB): calcd for (C55H45N4SiIr)+ m/z 982.3037; found m/z 982.3032. Reaction between Ir(ttp)Cl(CO) 1a and Diphenylsilane (7a). Diphenylsilane (1.6 mL, 500 equiv) and Ir(ttp)Cl(CO) (16.0 mg, 0.017 mmol) were heated at 140 °C for 2 days under N2. A purple solid of Ir(ttp)SiPh2H 7b (12.1 mg, 0.0116 mmol, 68%) was obtained after column chromatography. Rf ) 0.56 (hexane/CH2Cl2 ) 5:1). 1H NMR (CDCl3, 300 MHz): δ -1.76 (s, 1 H, 1JSi-H ) 204.6 Hz), 2.68 (s, 12 H), 4.50 (d, 4 H, J ) 7.5 Hz), 6.39 (t, 4 H, J ) 7.5 Hz), 6.75 (t, 2 H, J ) 7.5 Hz), 7.48 (t, 8 H, J ) 7.5 Hz), 7.72 (d, 4 H, J ) 7.5 Hz), 7.95 (d, 4 H, J ) 7.5 Hz), 8.45 (s, 8 H).

Li and Chan 13 C NMR (CDCl3, 75 MHz): δ 21.8, 124.5, 126.2, 127.5, 127.7, 131.4, 132.0, 133.8, 134.0, 137.4, 138.9, 143.2. HRMS (FAB): calcd for (C60H47N4SiIr)+ m/z 1044.3194; found m/z 1044.3209. Anal. Calcd for C60H47N4SiIr: C, 69.00; H, 4.54; N, 5.36. Found: C, 69.11; H, 4.54; N, 5.38. Reaction between Ir(ttp)Cl(CO) 1a and Diphenylmethylsilane (8a). Diphenylmethylsilane (1.5 mL, 500 equiv) and Ir(ttp)Cl(CO) (14.0 mg, 0.015 mmol) were heated at 200 °C for 6 h under N2. A purple solid of Ir(ttp)SiPh2Me 8b (6.7 mg, 0.0063 mmol, 42%) was obtained after column chromatography. Rf ) 0.51 (hexane/CH2Cl2 ) 5:1). 1H NMR (CDCl3, 300 MHz): δ -3.07 (s, 3 H), 2.68 (s, 12 H), 4.61 (d, 4 H, J ) 7.2 Hz), 6.46 (t, 4 H, J ) 7.5 Hz), 6.77 (t, 2 H, J ) 7.2 Hz), 7.46 (q, 8 H, J ) 7.5 Hz), 7.78 (d, 4 H, J ) 7.8 Hz), 7.89 (d, 4 H, J ) 7.8 z), 8.42(s, 8 H). 13C NMR (CDCl3, 75 MHz): δ -8.2, 21.8, 124.7, 126.1, 127.1, 127.6, 127.7, 131.4, 131.6, 133.6, 133.7, 133.9, 137.4, 138.9, 143.4. HRMS (FAB): calcd for (C61H49N4SiIr)+ m/z 1058.3350; found m/z 1058.3367. Anal. Calcd for C54H51N4O3SiIr: C, 69.22; H, 4.67; N, 5.29. Found: C, 69.21; H, 4.65; N, 5.03. Ir(ttp)SiPhMeH 6b (4.4 mg, 0.0045 mmol, 30%) was also generated according to the 1H NMR spectrum of the crude reaction mixture. Reaction of Ir(ttp)Cl(CO) 1a and Diphenylmethylsilane (8a). Diphenylmethylsilane (1.4 mL, 500 equiv) was added to Ir(ttp)Cl(CO) (13.4 mg, 0.014 mmol), and the mixture was degassed by the freeze-pump-thaw method (3 cycles). Then the mixture was heated at 200 °C under N2. The reaction mixture was monitored by 1H NMR spectroscopy by taking aliquots of the reaction mixture. After 1 day, the ratio of Ir(ttp)SiPh2Me 8b to Ir(ttp)SiPhMeH 6b was about 1.00:1.16. Reaction of Ir(ttp)Cl(CO) 1a with Silanes with Base Added. General Procedure. The reaction of Ir(ttp)Cl(CO) 1a with triethylsilane (2a) with addition of K3PO4 is described as a typical example. Triethylsilane (1.1 mL, 500 equiv) was added to a mixture of Ir(ttp)Cl(CO) (12.6 mg, 0.014 mmol) and K3PO4 (29.7 mg, 0.140 mmol, 10 equiv), and the mixture was degassed by the freezepump-thaw method (3 cycles). Then the mixture was heated at 140 °C for 1 day under N2. The crude product was purified by chromatography on alumina eluting with hexane. A purple solid of Ir(ttp)SiEt3 2b (12.0 mg, 0.0123 mmol, 88%) was produced. Reaction of Ir(ttp)Cl(CO) 1a with Triethylsilane (2a) with K3PO4. Triethylsilane (1.3 mL, 500 equiv) was added to a mixture of Ir(ttp)Cl(CO) (14.8 mg, 0.016 mmol) and K3PO4 (17.0 mg, 0.080 mmol, 5 equiv). Then the mixture was heated at 140 °C for 1 day under N2. A purple solid of Ir(ttp)SiEt3 2b (7.2 mg, 0.0074 mmol, 46%) was obtained after column chromatography. Reaction of Ir(ttp)Cl(CO) 1a with Triethylsilane (2a) with K3PO4. Triethylsilane (1.4 mL, 500 equiv) was added to a mixture of Ir(ttp)Cl(CO) (15.8 mg, 0.017 mmol) and K3PO4 (72.2 mg, 0.340 mmol, 20 equiv). Then the mixture was heated at 140 °C for 1 day under N2. A purple solid of Ir(ttp)SiEt3 2b (13.3 mg, 0.0136 mmol, 80%) was obtained after column chromatography. Reaction of Ir(ttp)Cl(CO) 1a with Triethylsilane (2a) with K2CO3. Triethylsilane (1.0 mL, 500 equiv) was added to a mixture of Ir(ttp)Cl(CO) (12.2 mg, 0.013 mmol) and K2CO3 (18.0 mg, 0.130 mmol, 10 equiv). Then the mixture was heated at 140 °C for 1 day under N2. A purple solid of Ir(ttp)SiEt3 2b (2.0 mg, 0.0021 mmol, 16%) was obtained after column chromatography. Reaction of Ir(ttp)Cl(CO) 1a with Triethylsilane (2a) with KOH. Triethylsilane (1.0 mL, 500 equiv) was added to a mixture of Ir(ttp)Cl(CO) (12.2 mg, 0.013 mmol) and KOH (7.3 mg, 0.130 mmol, 10 equiv). Then the mixture was heated at 140 °C for 1 day under N2. A purple solid of Ir(ttp)SiEt3 2b (4.7 mg, 0.0048 mmol, 37%) was obtained after column chromatography. Reaction of Ir(ttp)Cl(CO) 1a with Triethylsilane (2a) with KF. Triethylsilane (1.3 mL, 500 equiv) was added to a mixture of Ir(ttp)Cl(CO) (15.3 mg, 0.017 mmol) and KF (9.9 mg, 0.170 mmol, 10 equiv). Then the mixture was heated at 140 °C for 5 days under

Silicon-Hydrogen Bond ActiVation of Silanes N2. A purple solid of Ir(ttp)SiEt3 2b (8.3 mg, 0.0085 mmol, 50%) was obtained after column chromatography. Reaction of Ir(ttp)Cl(CO) 1a with Triethylsilane (2a) with CsCl. Triethylsilane (1.2 mL, 500 equiv) was added to a mixture of Ir(ttp)Cl(CO) (14.0 mg, 0.015 mmol) and CsCl (25.3 mg, 0.150 mmol, 10 equiv). Then the mixture was heated at 140 °C for 6 days under N2. A purple solid of Ir(ttp)SiEt3 2b (0.6 mg, 0.0006 mmol, 4%) was obtained after column chromatography. Reaction of Ir(ttp)Cl(CO) 1a with Triethylsilane (2a) with KI. Triethylsilane (1.1 mL, 500 equiv) was added to a mixture of Ir(ttp)Cl(CO) (13.2 mg, 0.014 mmol) and KI (23.2 mg, 0.140 mmol, 10 equiv). Then the mixture was heated at 140 °C for 5 days under N2. A purple solid of Ir(ttp)SiEt3 2b (1.9 mg, 0.0020 mmol, 14%) was obtained after column chromatography. Reaction of Ir(ttp)Cl(CO) 1a with Triethylsilane (2a) with KOAc. Triethylsilane (1.2 mL, 500 equiv) was added to a mixture of Ir(ttp)Cl(CO) (14.4 mg, 0.016 mmol) and KOAc (15.7 mg, 0.160 mmol, 10 equiv). Then the mixture was heated at 140 °C for 5 days under N2. A purple solid of Ir(ttp)SiEt3 2b (1.9 mg, 0.0019 mmol, 12%) was obtained after column chromatography. Reactions of Ir(ttp)Cl(CO) 1a with Silanes in Solvent. General Procedure. The reaction of Ir(ttp)Cl(CO) 1a with triethylsilane (2a) in benzene is described as a typical example. Triethylsilane (0.22 mL, 100 equiv) was added to Ir(ttp)Cl(CO) (12.9 mg, 0.014 mmol) in benzene (1.0 mL), and the mixture was degassed by the freeze-pump-thaw method (3 cycles). Then the mixture was heated at 200 °C for 9 days under N2. The crude product was purified by chromatography on alumina eluting with hexane. A purple solid of Ir(ttp)SiEt3 2b (7.9 mg, 0.0081 mmol, 58%) was produced. Reaction of Ir(ttp)Cl(CO) 1a with Triethylsilane 2a in Cyclohexane. Triethylsilane (0.22 mL, 100 equiv) was added to Ir(ttp)Cl(CO) (12.6 mg, 0.014 mmol) in cyclohexane (1.0 mL). Then the mixture was heated at 200 °C for 9 days under N2. A purple solid of Ir(ttp)SiEt3 2b (8.2 mg, 0.0084 mmol, 60%) was obtained after column chromatography. Reaction of Ir(ttp)Cl(CO) 1a with Triethoxylsilane (3a) in Benzene. Triethoxylsilane (0.24 mL, 100 equiv) was added to Ir(ttp)Cl(CO) (12.2 mg, 0.013 mmol) in benzene (1.0 mL). Then the mixture was heated at 200 °C for 6 h under N2. A purple solid of Ir(ttp)Si(OEt)3 3b (11.3 mg, 0.0110 mmol, 85%) was obtained after column chromatography. Reaction of Ir(ttp)Cl(CO) 1a with Triethoxylsilane (3a) in Cyclohexane. Triethoxylsilane (0.26 mL, 100 equiv) was added to Ir(ttp)Cl(CO) (12.9 mg, 0.014 mmol) in cyclohexane (1.0 mL). Then the mixture was heated at 200 °C for 12 h under N2. A purple solid of Ir(ttp)Si(OEt)3 3b (12.6 mg, 0.0123 mmol, 88%) was obtained after column chromatography. Reaction of Ir(ttp)Cl(CO) 1a with Triethylsilane (2a) in Benzene with K3PO4 Added. Triethylsilane (0.26 mL, 100 equiv) was added to a mixture of Ir(ttp)Cl(CO) (14.4 mg, 0.016 mmol) and K3PO4 (34.0 mg, 0.160 mmol, 10 equiv) in benzene (1.0 mL), and the mixture was degassed by the freeze-pump-thaw method (3 cycles). Then the mixture was heated at 140 °C for 3 days under N2. The crude product was purified by chromatography on alumina eluting with hexane. A purple solid of Ir(ttp)SiEt3 2b (14.6 mg, 0.015 mmol, 92%) was produced. Reaction of Ir(ttp)Cl(CO) 1a with Triethylsilane (2a) in Benzene-d6 with K3PO4 Added in a Sealed NMR Tube. Triethylsilane (90 µL, 100 equiv) was added to a mixture of Ir(ttp)Cl(CO) (5.4 mg, 0.006 mmol) and K3PO4 (12.3 mg, 0.058 mmol, 10 equiv) in benzene-d6 (0.5 mL), and the mixture was degassed by the freeze-pump-thaw method (3 cycles). Then the mixture was heated at 140 °C in the presence of light under vacuum. When the reaction was traced by 1H NMR, after 1.5 h, Ir(ttp)H was observed first without any Ir(ttp)SiEt3.

Organometallics, Vol. 27, No. 16, 2008 4041 Reaction of Ir(ttp)(BF4)(CO) 1c and Triethylsilane (2a)Triethylsilane (1.0 mL, 500 equiv) was added to Ir(ttp)(BF4)(CO) (12.8 mg,0.013mmol),andthemixturewasdegassedbythefreeze-pump-thaw method (3 cycles). Then the mixture was heated at 200 °C for 1 day under N2. The crude product was purified by chromatography on alumina eluting with hexane. A purple solid of Ir(ttp)SiEt3 2b (7.7 mg, 0.0079 mmol, 61%) was obtained. Reaction of Ir(ttp)H 1d and Triethylsilane (2a). Triethylsilane (1.3 mL, 500 equiv) was added to Ir(ttp)H (14.0 mg, 0.016 mmol), and the mixture was degassed by the freeze-pump-thaw method (3 cycles). Then the mixture was heated at 140 °C for 2 days under N2. The crude product was purified by chromatography on alumina eluting with hexane. A purple solid of Ir(ttp)SiEt3 2b (14.4 mg, 0.0147 mmol, 92%) was produced. Reaction of Ir(ttp)H 1d and Diphenylmethylsilane (8a). Diphenylmethylsilane (1.0 mL, 500 equiv) was added to Ir(ttp)H (9.0 mg, 0.010 mmol), and the mixture was degassed by the freeze-pump-thaw method (3 cycles). Then the mixture was heated at 200 °C under N2. The reaction mixture was monitored by 1H NMR spectroscopy by taking aliquots of the reaction mixture. After 2 h, the ratio of Ir(ttp)SiPh2Me 8b to Ir(ttp)SiPhMeH 6b was about 1.00:0.71. Reactions of Ir(ttp)Me 1b with Silanes. General Procedure. The reaction of Ir(ttp)Me 1b with triethylsilane (2a) is described as a typical example. Triethylsilane (1.0 mL, 500 equiv) was added to Ir(ttp)Me (11.1 mg, 0.013 mmol), and the mixture was degassed by the freeze-pump-thaw method (3 cycles). Then the mixture was heated at 200 °C for 5 days under N2. The crude product was purified by chromatography on alumina eluting with hexane. A purple solid of Ir(ttp)SiEt3 2b (10.5 mg, 0.0108 mmol, 83%) was obtained. Reaction between Ir(ttp)Me 1b and Triethylsilane (2a). Triethylsilane (1.2 mL, 500 equiv) and Ir(ttp)Me (13.4 mg, 0.015 mmol) were heated at 150 °C for 8 days under N2. A purple solid of Ir(ttp)SiEt3 2b (1.8 mg, 0.0018 mmol, 12%) was obtained after column chromatography. Reaction between Ir(ttp)Me 1b and Triethoxysilane (3a). Triethoxysilane (1.1 mL, 500 equiv) and Ir(ttp)Me (10.9 mg, 0.012 mmol) were heated at 200 °C for 7 days under N2. A purple solid of Ir(ttp)Si(OEt)3 3b (2.0 mg, 0.0019 mmol, 16%) was obtained after column chromatography. Reactions of Ir(ttp)Me 1b with Silanes in Benzene. General Procedure. The reaction of Ir(ttp)Me 1b with triethylsilane (2a) in benzene is described as a typical example. Triethylsilane (0.24 mL, 100 equiv) was added to Ir(ttp)Me (12.9 mg, 0.015 mmol) in benzene (1.0 mL), and the mixture was degassed by the freeze-pump-thaw method (3 cycles). Then the mixture was heated at 200 °C for 7 days under N2. The crude product was purified by chromatography on alumina eluting with hexane. A purple solid of Ir(ttp)SiEt3 2b (1.5 mg, 0.0015 mmol, 10%) was produced. Reaction of Ir(ttp)Me 1b with Triethoxylsilane (3a) in Benzene. Triethoxylsilane (0.26 mL, 100 equiv) was added to Ir(ttp)Me (11.9 mg, 0.014 mmol) in benzene (1.0 mL). Then the mixture was heated at 200 °C for 4 days under N2. A purple solid of Ir(ttp)Si(OEt)3 3b (1.1 mg, 0.0011 mmol, 8%) was obtained after column chromatography. Reaction of Ir(ttp)Me 1b with Triethylsilane (2a) with Base Added. General Procedure. The reaction of Ir(ttp)Me 1b with triethylsilane (2a) with addition of K3PO4 is described as a typical example. Triethylsilane (1.2 mL, 500 equiv) was added to a mixture of Ir(ttp)Me (13.1 mg, 0.015 mmol) and K3PO4 (31.8 mg, 0.150 mmol, 10 equiv), and the mixture was degassed by the freeze-pump-thaw method (3 cycles). Then the mixture was heated at 150 °C for 8 days under N2. The crude product was

4042 Organometallics, Vol. 27, No. 16, 2008 purified by chromatography on alumina eluting with hexane. A purple solid of Ir(ttp)SiEt3 2b (6.4 mg, 0.0066 mmol, 44%) was produced. Reaction of Ir(ttp)Me 1b with Triethylsilane (2a) with KOH. Triethylsilane (1.1 mL, 500 equiv) was added to a mixture of Ir(ttp)Me (12.3 mg, 0.014 mmol) and KOH (7.8 mg, 0.140 mmol, 10 equiv). Then the mixture was heated at 150 °C for 8 days under N2. Only a trace amount of Ir(ttp)SiEt3 2b was observed according to 1H NMR. Reaction of Ir(ttp)Me 1b with Triethylsilane (2a) with K2CO3. Triethylsilane (1.0 mL, 500 equiv) was added to a mixture of Ir(ttp)Me (11.8 mg, 0.013 mmol) and K2CO3 (18.0 mg, 0.130 mmol, 10 equiv). Then the mixture was heated at 150 °C for 8 days under N2. A purple solid of Ir(ttp)SiEt3 2b (1.8 mg, 0.0018 mmol, 14%) was produced. Reaction of Ir(ttp)Me 1b with Triethylsilane (2a) with KF. Triethylsilane (1.2 mL, 500 equiv) was added to a mixture of Ir(ttp)Me (12.8 mg, 0.015 mmol) and KF (8.7 mg, 0.150 mmol, 10 equiv). Then the mixture was heated at 150 °C for 8 days under N2. A purple solid of Ir(ttp)SiEt3 2b (2.2 mg, 0.0023 mmol, 15%) was produced. Reaction of Ir(ttp)Me 1b with Triethylsilane (2a) with CsCl. Triethylsilane (1.3 mL, 500 equiv) was added to a mixture of Ir(ttp)Me (14.1 mg, 0.016 mmol) and CsCl (26.9 mg, 0.160 mmol, 10 equiv). Then the mixture was heated at 150 °C for 8 days under N2. A purple solid of Ir(ttp)SiEt3 2b (0.9 mg, 0.0010 mmol, 6%) was produced. Reaction of Ir(ttp)Me 1b with Triethylsilane (2a) with KI. Triethylsilane (1.1 mL, 500 equiv) was added to a mixture of Ir(ttp)Me (12.1 mg, 0.014 mmol) and KI (23.2 mg, 0.140 mmol, 10 equiv). Then the mixture was heated at 150 °C for 8 days under N2. A purple solid of Ir(ttp)SiEt3 2b (1.2 mg, 0.0013 mmol, 9%) was produced. Reaction of Ir(ttp)Me 1b with Triethylsilane (2a) with KOAc. Triethylsilane (1.2 mL, 500 equiv) was added to a mixture of Ir(ttp)Me (13.3 mg, 0.015 mmol) and KOAc (14.7 mg, 0.150 mmol, 10 equiv). Then the mixture was heated at 150 °C for 8 days under N2. A purple solid of Ir(ttp)SiEt3 2b (8.8 mg, 0.0090 mmol, 60%) was produced. Reaction of Ir(ttp)SiPh2Me (8b) with Diphenylmethylsilane (8a). Diphenylmethylsilane (1.2 mL, 500 equiv) was added to Ir(ttp)SiPh2Me (12.2 mg, 0.012 mmol), and the mixture was

Li and Chan degassed by the freeze-pump-thaw method (3 cycles). Then the mixture was heated at 200 °C for 2 days under N2. The crude product was purified by chromatography on alumina eluting with hexane. A purple solid of Ir(ttp)SiPh2Me 8b (4.2 mg, 0.0040 mmol, 33%) was obtained, and a 31% yield of Ir(ttp)SiPhMeH 6b was produced according to 1H NMR of the crude mixture. Reaction of Ir(ttp)SiPhMeH (6b) with Diphenylmethylsilane (8a). Diphenylmethylsilane (1.5 mL, 500 equiv) was added to Ir(ttp)SiPhMeH (14.6 mg, 0.015 mmol), and the mixture was degassed by the freeze-pump-thaw method (3 cycles). Then the mixture was heated at 200 °C for 2 days under N2. The crude product was purified by chromatography on alumina eluting with hexane. A purple solid of Ir(ttp)SiPh2Me 8b (6.4 mg, 0.0060 mmol, 40%) was produced, and 46% recovery yield of Ir(ttp)SiPhMeH 6b was obtained according to 1H NMR of the crude mixture. Reaction of Ir(ttp)SiEt3 2b with Benzyldimethylsilane (4a). Benzyldimethylsilane (1.0 mL, 500 equiv) was added to Ir(ttp)SiEt3 (12.2 mg, 0.012 mmol), and the mixture was degassed by the freeze-pump-thaw method (3 cycles). Then the mixture was heated at 200 °C for 2 days under N2. The crude product was purified by chromatography on alumina eluting with hexane. A purple solid of Ir(ttp)SiBnMe2 4b (9.9 mg, 0.0098 mmol, 82%) was produced. Reaction of Ir(ttp)SiBnMe2 4b with Triethylsilane (2a). Triethylsilane (1.2 mL, 500 equiv) was added to Ir(ttp)SiBnMe2 (15.2 mg, 0.015 mmol), and the mixture was degassed by the freeze-pump-thaw method (3 cycles). Then the mixture was heated at 200 °C for 4 days under N2, and 4b was not consumed completely. The crude product was purified by chromatography on alumina eluting with hexane. A purple solid of Ir(ttp)SiEt3 2b (2.9 mg, 0.0030 mmol, 20%) was produced, and Ir(ttp)SiBnMe2 4b (10.8 mg, 0.0106 mmol, 71%) was recovered.

Acknowledgment. We thank the Research Grants Council of Hong Kong of the SAR of China for financial support (No. 400105). Supporting Information Available: Crystallographic data for 3b and 4b and spectra for 5b and 6b. This material is available free of charge via the Internet at http://pubs.acs.org. OM701144A